Black ceramic sinter with low thermal expansion and high specific rigidity and process for producing the same

ABSTRACT

This invention provides a black low thermal expansion high specific rigidity ceramic sintered body having a black tone, manifesting very small thermal expansion at room temperature and abounding in rigidity and specific rigidity, and a method for the production thereof. The black low thermal expansion high specific rigidity ceramic sintered body is characterized by having a chemical composition comprising 8.0–17.2 mass % of MgO, 22.0–38.0 mass % of Al 2 O 3 , 49.5–65.0 mass % of SiO 2 , a total of 0.1–2 mass % of one or more transition elements as reduced to oxides, and 0–2.5 mass % of Li 2 O, and having the mass ratios satisfy the relationships of (SiO 2 −8×Li 2 O)/MgO≧3.0 and (SiO 2 −8×Li 2 O)/Al 2 O 3 ≧1.2. The method for the production of a black low thermal expansion high specific rigidity ceramic sintered body of this invention is characterized by forming the sintered body in an atmosphere of a non-oxidizing gas at a temperature in the range of 1200–1500° C.

TECHNICAL FIELD

This invention relates to a black low thermal expansion high specificrigidity ceramic sintered body possessing a black tone, manifesting verysmall thermal expansion at room temperature, and abounding in rigidityand specific rigidity, and to a method for the production thereof. Moreparticularly, this invention relates to a black low thermal expansionhigh specific rigidity ceramic sintered body for the use in membersincluding production units (such as an exposure meter, a processingmachine, and construction machine) for semiconductors and magnetic headswhich abhor such phenomena as thermal deformation of a device caused byfluctuation of an ambient temperature or emission of heat from thedevice itself and unnecessary reflection (including irregularreflection) or transmission of light and measuring devices, and to amethod for the production thereof.

BACKGROUND ART

Recently, owing to the trend of semiconductors toward higher integrationand magnetic heads toward further miniaturization, the production units(such as an exposure meter, a processing machine, and a constructionmachine) for such semiconductors and magnetic heads, measuring devices,measuring prototypes, and reflecting mirrors have reached the point ofrequiring high dimensional accuracy and high rigidity. For thesedevices, the stability of dimensional accuracy has also come to gaininsignificance. The prevention of such devices from incurring thedeformation which is caused by the fluctuation of an ambient temperatureor the emission of heat from the device itself has become an importanttask. The materials which produce very small thermal expansions andabound in rigidity and specific rigidity (Young's modulus/specificgravity) have come to find the use for component members in suchdevices.

The various devices as mentioned above are mostly aimed at handling thelights such as laser beam, ultraviolet light, and visible ray for thepurpose of exposure or measurement. The members which are used in thesedevices more often than not abhor unnecessary reflection (includingirregular reflection) or transmission of light. On many occasions, blackmaterials which succumb only sparingly to reflection and transmission oflight are found as necessary supplies.

The low thermal expansion materials which have heretofore known in theart include Invar alloy (Fe—Ni type) and super-Invar alloy (Fe—Ni—Cotype) in the class of metals, such low thermal expansion glasses asZERODUR™ glass (available from Schott ML GmbH in Germany), quartz glass(SiO₂), and titanium dioxide-containing quartz glass (SiO₂—TiO₂) in theclass of ceramics, aluminum titanate (TiO₂—Al₂O₃), cordierite(MgO—Al₂O₃—SiO₂) type sintered body and glass, lithium-alumino-silicate(Li₂O—Al₂O₃—SiO₂) type sintered body and glass.

The super-Invar alloy as a low thermal expansion metal indeed manifestssuch a low thermal expansion coefficient as 0.13×10⁻⁶/° C. at roomtemperature and yet has such a large specific gravity as 8.2 g/cm² andsuch a not very high Young's modulus as 125 GPa. Thus, it has a very lowspecific rigidity approximately of 15 GPa cm³/g and, therefore, isdeficient in mechanical stability. The term “specific rigidity” as usedherein means a magnitude obtained by dividing a Young's modulus (E) by aspecific gravity (ρ) (specific rigidity=E/ρ). The impartation of theblack color to the surface of this alloy has no alternative but to relyon the method of using such a surface coat as a black Cr plating on thesurface layer. The use of the surface coat, however, entails the problemof exerting adverse effects on the low thermal expansion property andthe precision machining property.

The quartz glass, while enjoying such a low thermal expansioncoefficient as 0.48×10⁻⁶/° C., suffers from such an insufficientspecific rigidity as about 33 GPa cm³/g and a clear tone.

The ZERODUR™ glass has been finding the extensive utility inapplications to such devices as measuring prototypes. It enjoys such asufficiently low thermal expansion coefficient as 0.02×10⁻⁶/° C. at roomtemperature and yet suffers from a clear tone. It further encountersdifficulty in forming products too complicate in shape and products toolarge in size to manufacture. Further, since it manifests specificrigidity and Young's modulus respectively approximating 35.6 GPa·cm³/gand 90 GPa, it does not fully fit the use aimed at by this invention.

As regards the aluminum titanate, it has been known to have produced asintered body manifesting such a low thermal expansion coefficient as−0.068×10⁻⁶/° C. (“Glossary of Fine Ceramics Catalogs (1987)”, p. 140).This compound manifests such a high water absorption as 1.59% and thusoffers only insufficient denseness for the use aimed at by thisinvention. No sintered bodies of this compound has been known to possessa black tone.

The lithium-alumino-silicate type sintered body and glass are deficientin mechanical stability because it manifests only such insufficientlyhigh specific rigidity as not more than 33 GPa·cm³/g in spite of such asmall thermal expansion coefficient as falls in the range of −5 to1×10⁻⁶/° C. It has predominantly acquired a white tone and has notacquired a black tone so far.

JP-A-61-72, 679 discloses a low thermal expansion ceramic sintered bodywhich has a chemical composition mainly comprising 0.3–5.5 mass % ofLi₂O, 4.1–16.4 mass % of MgO, 2.07–42.8 mass % of Al₂O₃, and 46.3–70.1mass % of SiO₂, a crystal phase containing not less than 30 mass % ofcordierite and not less than 5 mass % of β-spodumene as main components,and manifests a thermal expansion coefficient of 2.0×10⁻⁶/° C. at atemperature in the range of 20° C.–800° C. This publication, however,has absolutely no mention of the tone of the sintered body and pointsout the fact that the sintered body produced by the method taught in thepublication does not acquire a black tone (refer to Comparative Example22 in Table 1 inserted in the working example which will be specificallydescribed hereinbelow).

JP-A-10-53,460 reports a dense ceramic substance which comprises 1.5–6.5mass % of Li₂O, 1.0–10 mass % of MgO, 14–30 mass % of Al₂O₃, and 58–83mass % of SiO₂, and allows the coexistence of petalite, spodumene, andcordierite in a crystal phase and demonstrates that this substanceexcels in resistance to thermal shock. This publication, however, hasabsolutely no mention of the tone of the ceramic and points out the factthat the sintered body produced by the method taught in the publicationdoes not acquire a black tone (refer to Comparative Example 23 in Table1 inserted in the working example which will be specifically describedhereinbelow).

“Ceramics”, Vol. 18 (1983) No. 5 discloses a Co—Cr—Fe type spinel, aCo—Mn—Fe type spinel, a Co—Mn—Cr—Fe type spinel, a Co—Ni—Cr—Fe typespinel, and a Co—Ni—Mn—Cr—Fe type spinel as black pigments for the usein coloring ceramics and also discloses a solid solution of Sb in SiO₂and a solid solution of Co and Ni in ZrSiO₄ as gray pigments. Thesepigments, however, are intended to utilize the phenomenon of colorationin the graze on the surface of ceramics and not to impart a black colorto a depth in the sintered body itself. Any attempt to use the graze onthe surface of a low thermal expansion ceramic substance proves futilebecause the difference in thermal expansion between the ceramicsubstance and the graze tends to inflict a crack to the applied layer ofthe graze.

The silicon carbide sintered body has been commercially available as ablack ceramic substance. JP-A-08-310,860 discloses a black zirconiaceramic sintered body, JP-A-04-50,161 a method for the production of ahigh rigidity black alumina sintered body, and JP-A-06-172,034 a blacksilicon nitride sintered body, respectively. Though these sinteredbodies are black, their thermal expansion coefficients at roomtemperature are 2.3×10⁻⁶/° C. in the sintered body of silicon carbide,7×10⁻⁶/° C. in that of zirconia, 5.3×10⁻⁶/° C. in that of alumina, and1.3×10⁻⁶/° C. in that of silicon nitride. Thus, these sintered bodiesare incapable of realizing a low thermal expansion coefficient aimed atthis invention.

Incidentally, the term “room temperature” as used in this inventionrefers to the range of temperatures, 20° C.–25° C. The room temperaturementioned in the present specification invariably refers to _(this)temperature range.

JP-B-57-29,436 discloses a technique which comprises adding to acordierite sintered body an oxide of such a transition element as Zn, V,Nb, Cr, Mo, or W for the purpose of densifying the sintered body. Thesintered body obtained by this technique, however, manifests such aninsufficiently low thermal expansion coefficient as 0.96×10⁻⁶/° C.,fails to acquire sufficient densification as evident from waterabsorption of 4.6%, and suffers from not sufficiently high rigidity. Thepublication has absolutely no mention of the tone.

Recently, JP-A-11-343,168 discloses a technique for the impartation of ablack color to a ceramic substance containing not less than 80 mass % ofcordierite by the incorporation of 0.1–2.0 mass % of carbon into theceramic substance.

The invention disclosed in this publication is characterized byincorporating carbon and, therefore, is different from the presentinvention which does not need the incorporation of carbon.

The incorporation of carbon in a sintered body entails the problem ofheightening the thermal expansion coefficient as indicated in the abovepublication, exerts such an adverse effect on mechanical properties aslowering the modulus of elasticity, renders the formation of productslarge in wall thickness and size difficult to attain by sintering, andinevitably imposes restrictions on the shapes of such products. Thus,the incorporation of carbon proves unfavorable.

The material which contains no carbon, assumes a black color, manifestslow thermal expansion, and possesses rigidity and specific rigidity highenough to ensure effective use as building materials has not been knownto date.

This invention is aimed at providing a black low thermal expansionceramic sintered body which assumes a black tone and manifests very lowthermal expansion and high rigidity and specific rigidity at roomtemperature and a method for the production thereof.

DISCLOSURE OF THE INVENTION

To be specific, this invention is featured by the following items.

-   (1) A black low thermal expansion high specific rigidity ceramic    sintered body, characterized by having a thermal expansion    coefficient of not more than 0.6×10⁻⁶/° C. in absolute value at room    temperature, a modulus of elasticity (Young's modulus) of not less    than 100 GPa, and specific rigidity (Young's modulus/specific    gravity) of not less than 40 GPa·cm³/g and assuming a black tone.-   (2) A black low thermal expansion high specific rigidity ceramic    sintered body, characterized by having a chemical composition    comprising 8.0–17.2 mass % of MgO, 22.0–38.0 mass % of Al₂O₃,    49.5–65.0 mass % of SiO₂, a total of 0.1–2 mass % of one or more    transition elements as reduced to oxides, and 0–2.5 mass % of Li₂O,    and having the mass ratios satisfy the relationships of    (SiO₂−8×Li₂O)/MgO≧3.0 and (SiO₂−8×Li₂O)/Al₂O₃≧1.2.-   (3) A black low thermal expansion high specific rigidity ceramic    sintered body according to the item (2), wherein the thermal    expansion coefficient is not more than 0.6×10⁻⁶/° C. in absolute    value at room temperature, the modulus of elasticity (Young's    modulus) is not less than 100 GPa, and the specific rigidity    (Young's modulus/specific gravity) is not less than 40 GPa·cm³/g and    the tone of the sintered body is black.-   (4) A black low thermal expansion high specific rigidity ceramic    sintered body according to any one of the items (1) through (3),    wherein the total reflectivity of the sintered body is not more than    17% at a wavelength of light in the range of 200–950 nm.-   (5) A black low thermal expansion high specific rigidity ceramic    sintered body according to any one of the items (1) through (4),    wherein the apparent porosity of the sintered body is not more than    2%.-   (6) A black low thermal expansion high specific rigidity ceramic    sintered body according to any one of the items (1) through (5),    wherein not less than 80 vol. % of the crystal phase of the sintered    body is a crystal phase of cordierite.-   (7) A black low thermal expansion high specific rigidity ceramic    sintered body according to any one of the items (1) and (3) through    (6), wherein the thermal expansion coefficient is not more than    0.3×10⁻⁶/° c. in absolute value at room temperature.-   (8) A black low thermal expansion high specific rigidity ceramic    sintered body according to any one of the items (1) and (3) through    (7), wherein the modulus of elasticity is not less than 120 GPa and    the specific rigidity is not less than 50 GPa·cm³/g.-   (9) A black low thermal expansion high specific rigidity ceramic    sintered body according to any one of the items (2) through (8),    wherein the chemical composition has such mass ratios as satisfy the    relationships of (SiO₂−8×Li₂O)/MgO≧3.65 and (SiO₂−8×Li₂O)/Al₂O₃≧1.3.-   (10) A method for the production of a black low thermal expansion    high specific rigidity ceramic sintered body, characterized by    forming the sintered body in an atmosphere of a non-oxidizing gas at    a temperature in the range of 1200–1500° C.-   (11) A method for the production of a black low thermal expansion    high specific rigidity ceramic sintered body set forth in any one of    the items (2) through (9), wherein the sintered body is formed in an    atmosphere of a non-oxidizing gas at a temperature in the range of    1200–1500° C.-   (12) A method for the production of a black low thermal expansion    high specific rigidity ceramic sintered body according to in the    item (10) or (11), wherein the non-oxidizing gas is one or more    members selected among argon, helium, nitrogen and hydrogen.-   (13) A method for the production of a black low thermal expansion    high specific rigidity ceramic sintered body according to any one of    the items (10) through (12), wherein the raw material powder is one    or more members selected from the group consisting of cordierite    powder, talc, magnesia spinel, magnesia, magnesium hydroxide,    magnesium carbonate, Li₂O—Al₂O₃—SiO₂ type powders (petalite,    spodumene, and eucriptite), lithium hydroxide, lithium carbonate,    alumina powder, silica powder, kaolin powder, and mullite powder.-   (14) A method for the production of a black low thermal expansion    high specific rigidity ceramic sintered body according to the item    (13), wherein not less than 70 mass % of the MgO component as the    MgO-source raw material is supplied by one or more members selected    from the group consisting of electro-molten cordierite powder,    synthetic cordierite powder, and talc powder.-   (15) A method for the production of a black low thermal expansion    high specific rigidity ceramic sintered body according to any one of    the items (10) through (14), wherein the sintering method is a hot    press method, an HIP method, a gas pressure sintering method, or a    normal pressure sintering method.

The sintered body of this invention can, by having a chemicalcomposition mainly comprise 8.0–17.2 mass % of MgO, 22.0–38.0 mass % ofAl₂O₃, and 49.5–65.0 mass % of SiO₂ and consequently forming a crystalcomposition mainly as a cordierite crystal phase, obtain a low thermalexpansion coefficient and a high specific rigidity which are aimed at bythis invention.

When the acquisition of the low thermal expansion coefficient at roomtemperature is not aimed at as in the present invention, Li₂O does notconstitute itself an essential component. The incorporation of Li₂O,however, may bring about such effects as enhancing the sinteringproperties, facilitating the formation of a dense sintered body,allowing the activation of the reaction (mass transfer) in the sinteredbody in the process of formation to start at a relatively lowtemperature, enabling the impartation of a black color to start at astill lower temperature, and causing the finally produced sintered bodyto be further blackened.

The sintered body of this invention can, by incorporating a total of0.1–2 mass % of one or more transition elements as reduced to oxides,having the mass ratios in a chemical composition satisfy therelationship: X=(SiO₂−8×Li₂O)/MgO≧3.0 and Y=(SiO₂−8×Li₂O)/Al₂O₃≧1.2, andeffecting the sintering process in an atmosphere of a non-oxidizing gasat a temperature in the range of 1200–1500° C., acquire a black tonewhich is aimed at by this invention.

BEST MODE OF EMBODYING THE INVENTION

For the quantitative determination about whether a given sintered bodyhas a black tone or not, a method which relies on the color differencefound with a calorimeter to attain the expected expression or a methodwhich utilizes the total reflectivity specified in JIS K7105 may beadopted. The applications to be found for the sintered body of thisinvention are such that they abhor the reflection of light includingirregular reflection and, therefore, are most appropriately rated forthe degree of this abhorrence by utilizing the total reflectivity. Thetotal reflectivity is attained by combined determination of directreflection and diffused reflection with the aid of a globularintegrating sphere. The determination in this case is made in accordancewith JIS K7105.

Generally, for such members used in devices handling a laser beam or anultraviolet light and requiring a black tone, products plated with blackchrome and products treated with black alumite may be adopted. The totalreflectivity in the products plated with black chrome is in the range of5–7% at a wavelength of light in the range of 200–950 nm and in theproducts treated with black alumite in the range of 6–8% at a wavelengthof light in the range of 200–650 nm or in the range of 10–60% at awavelength of light in the range of 700–950 nm. The total reflectivityis preferred to be as low as permissible because the degree with whichthe reflection is prevented is heightened in proportion as the totalreflectivity is lowered. Generally, so long as the total reflectivity isnot more than 17% within the range, 200–950 nm, of wavelength of light,members for such devices as abhorring the reflection of light can besafely used from the practical point of view. Better results can beobtained when the total reflectivity is not more than 12%.

In this invention, the sintered body of a black tone which is aimed atby this invention is characterized by having incorporated therein as acoloring auxiliary one or more transition elements in a total amount of0.1–2 mass % as reduced to oxides, having the mass ratios of thechemical composition satisfy the relationship: X=(SiO₂−8×Li₂O)/Mgo≧3.0and Y=(SiO₂−8×Li₂O)/Al₂O₃≧1.2, and performing the sintering process inan atmosphere of a non-oxidizing gas at a temperature in the range of1200–1500° C. The fulfillment of all these requirements enables thetotal reflectivity of the sintered body to fall in the rangecontemplated by this invention.

No fully satisfactory black tone can be obtained when the total amountof transition elements as coloring auxiliary is not more than 0.1 mass %as reduced to oxides. If the total amount is not less than 2 mass %, theexcess would be at a disadvantage in compelling the produced sinteredbody to form a low melting compound therein and giving rise to aphenomenon of foaming and bringing about the degradation of density andrigidity. The total amount of transition elements as reduced to oxidesis more preferably in the range of 0.3–1 mass %. By having the totalamount fall in this range, it is made possible to obtain a sintered bodyhaving sufficient blackness and abounding in density and rigidity.

As the transition elements which are usable in this invention, suchtransition (metal) elements as Cr, Mn, Fe, Co, Ni, and Cu prove mostadvantageous among other transition elements.

When the amount of SiO₂ decreases in the mass percentage composition,MgO—Al₂O₃—SiO₂, as the main component of the sintered body, the blackcolor imparted to the sintered body is proportionately lightened. TheLi₂O included in the composition, while the composition is in theprocess of forming a sintered body, fixes a part of SiO₂ in the form ofa Li₂O—Al₂O₃O—SiO₂ sintered body. When the percentage composition isconsidered, therefore, it requires subtraction of that part of theamount of SiO₂ from the SiO₂ amount. Empirically, it is inferred thatthe amount of SiO₂ so subtracted is approximately represented by 8×Li₂O.Accordingly, for the purpose of obtaining the black colorationcontemplated by this invention, it is necessary to transform the massratio in the chemical composition to this composition ratio,X=(SiO₂−8×Li₂O)/MgO≧3.0 and Y=(SiO₂−8×Li₂O)/Al₂O₃≧1.2, preferablyX=(SiO₂−8×Li₂O)/MgO≧3.65 and Y=(SiO₂−8×Li₂O)/Al₂O₃≧1.3. The mechanismresponsible for imparting a fully satisfactory black tone to a sinteredbody in consequence of securing the composition ratio of SiO₂ asdescribed above has not yet been fully elucidated. It may be logicallyexplained, however, by a postulate that the condition of the deficiencyof oxygen in the Si—O bond present in a crystal constitutes itself afactor of some sort or other.

For the sake of imparting a black tone to a given sintered body, thesintering atmosphere and the sintering temperature adopted for thesintered body form important factors. By the sintering operation in airwhich has been adopted in the ordinary sintering, the colorationattained at all is only in a light gray color or a blue color. Only byforming a sintered body in a non-oxidizing atmosphere at a temperaturein the range of 1200–1500° C., it is made possible to have the producedsintered body to assume a black tone. As regards the mechanismresponsible for the assumption of the black tone in such an atmosphereat such a temperature as specified above, it is inferred that the lackof oxygen likewise constitutes itself a factor of some sort or other.

For the sintering atmosphere, such a non-oxidizing gas as argon, helium,nitrogen, and hydrogen may be used. The sintering may be performed in areducing atmosphere formed by having a hydrogen gas partly incorporatedin such an inert gas as argon, which results in enhancing theimpartation of a black tone.

As respects the sintering temperature, if it is less than 1200° C., theshortage would be at a disadvantage in rendering it difficult to obtaina fully densified sintered body and, even when the sintered body isdensified at all, preventing this sintered body from thoroughlyundergoing coloration in black. If the temperature exceeds 1500° C., theexcess would be at a disadvantage in suffering the crystal phase of aproduced sintered body to melt and disrupting any plan to obtain anormal sintered body on account of such phenomena as fusion andexpansion. The sintering temperature is more preferably in the range of1275° C.–1450° C.

Regarding the thermal expansion coefficient of the sintered body of thisinvention, the thermal expansion coefficient is required to be not morethan 0.6×10⁻⁶/° C. in absolute value at room temperature in the light ofthe necessity for maintaining the dimensional accuracy and the stabilityneeded in production units for the recent high integrationsemiconductors and miniaturized magnetic heads. The precision memberswhich demand thermal stability of still higher accuracy are in need ofthermal expansion coefficients approximating closely to zero expansion.The thermal expansion coefficient is preferred to be not more than0.3×10⁻⁶/° C. (namely −0.3–0.3 10⁻⁶/° C.) in absolute value at roomtemperature. Here, the minus numerical value in the thermal expansioncoefficient means the fact that the relevant member shrinks as thetemperature rises. In the narrow temperature range around roomtemperature, a sintered body having minus thermal expansion coefficientwhile having the composition conforming to this invention may beobtained.

Concerning the rigidity (Young's modulus) of a sintered body, in orderfor the sintered body to serve effectively as a precision structurewithin a fixed space, the rigidity is required to have a Young's modulusof not less than 100 GPa, optimally of not less than 120 GPa. If Young'smodulus is less than 100 GPa, the shortage would be at a disadvantage inrequiring the structure to increase in wall thickness and size with aview to repressing the deformation of the relevant member.

Where a given sintered body is to be used in such a partial supportingmember as an end face supporting shaft, the specific rigidity (Young'smodulus/specific gravity) must be also high in order for the sinteredbody to retain precision fully sufficient for a structure. In thisinvention, the specific rigidity is required to be not less than 40GPa·cm³/g, preferably not less than 50 GPa·cm³/g.

In the sintered body of this invention, by being provided with achemical composition mainly comprising 8.0–17.2 mass % of MgO, 22,0–38.0mass % of Al₂O₃, 49.5–65.0 mass % of SiO₂, a crystal phase of thesintered body can be formed as a crystal phase made mainly of cordieriteand the thermal expansion coefficient in absolute value at roomtemperature, the modulus of elasticity (Young's modulus), and thespecific rigidity (Young's modulus/specific gravity) can be adjustedwithin the respective ranges contemplated by this invention. Though Li₂Ois not an essential component for the sintered body, the sinteringproperties may be enhanced and the impartation of a black tone may bepromoted by the incorporation of Li₂O.

The chemical composition of MgO, Al₂O₃, and SiO₂ is so fixed as to fallin the percentage composition as mentioned above with a view to enablingthe sintered body to acquire a crystal phase formed mainly ofcordierite. If the proportions of MgO and Al₂O₃ are unduly large, theexcess would be at a disadvantage in suffering the crystal phase ofspinel, mullite, or forsterite to acquire an unduly large thermalexpansion coefficient and the thermal expansion coefficient of thesintered body as a whole to exceed 0.6×10⁻⁶/° C. Conversely, if theproportions of MgO and Al₂O₃ are unduly small, the shortage would be ata disadvantage in unduly decreasing the crystal phase of cordierite. Ifthe proportion of SiO₂ is unduly large, the excess would induce adecrease in the modulus of elasticity. If this proportion is undulysmall, the shortage would unduly increase such crystal as spinel,mullite, or forsterite which has high thermal expansion coefficient.

The sintered body of this invention is enabled to be improved in thesintering properties by incorporating therein Li₂O in addition to MgO,Al₂O₃, and SiO₂. This improvement results in facilitating the formationof a densified sintered body. The start of the activation of thereaction (mass transfer) in the sintered body from a relatively lowtemperature onward can bring about such effects as enabling theimpartation of a black color to start at a low temperature and thefinally obtained sintered body to assume a black tone to a greaterextent. The addition of Li₂O in any amount exceeding 2.5 mass % isunfavorable because the excess would bring about a conspicuous decreasein the modulus of elasticity. For the purpose of deriving expectedeffects from the addition of Li₂O, this compound is preferred to beadded in an mount of not less than 0.1 mass %. By adjusting the amountof Li₂O to be added in the range of 0.2–1.0 mass %, it is made possibleto obtain a sintered body having such an extremely low thermal expansioncoefficient as of not more than 0.1×10⁻⁶/° C. in absolute value.

In this invention, for the purpose of acquiring the satisfactory lowthermal expansion coefficient and rigidity, the crystal phase ofcordierite in the sintered body is preferred to account for not lessthan 80 vol. %, optically not less than 90 vol. %, of the whole volumeof the sintered body. The expression “crystal phase of cordierite” asused in this invention means a crystal phase which comprises purecordierite crystal plus a crystal phase which, on X ray diffraction,manifests a diffraction peak of cordierite and yet reveals a change inthe lattice constant owing to a solid solution of Li and transitionelements.

The crystal phase of the sintered body of this invention may be asingle-phase crystal phase of cordierite or may additionally incorporatetherein a Li₂O—Al₂O₃—SiO₂ type crystal phase (β-spodumene, eucriptite,petalite). Any crystal phase other than these crystal phases ispreferred not to be contained in the sintered body from the viewpoint ofacquiring a low thermal expansion coefficient, though the incorporationthereof can be tolerated when the content thereof is not more than 5mass %.

Of the sintering method used for the production of the sintered body ofthis invention, such elements as the sintering atmosphere and thesintering temperature have been already described above.

The sintering method itself may be selected among a hot press method, ahot isostatic press (HIP) method, a gas pressure sintering method, and anormal pressure sintering method. The hot press method, the HIP method,and the gas pressure sintering method prove particularly favorable in asense that they prevent low melting substances from foaming and enablethe sintering process to perform at a higher temperature and they permita sintered body to be formed within a percentage composition in whichthe normal pressure sintering method fails to produce requiredsintering. Economically, the normal pressure sintering method excels allthe other methods enumerated above and can be applied to memberscomplicate and large. Thus, the sintering method can be used as variedwith the kind of application to be adopted.

Since the HIP method and the hot press method are capable of producingporeless materials (materials of specular surfaces), they can be appliedto the production of materials for reflecting mirrors, fastenermaterials which abhor the deposition of dirt in pores, andtransportation grade band members.

Since the sintered body of this invention is intended for such uses ascherish precision, it is not allowed to change size and geometricprecision over time and to generate an outer gas. The sintered body,therefore, is required to have a dense texture. For this reason, theapparent porosity is required to be not more than 2%, preferably notmore than 0.2%. Even for the sake of securing the rigidity contemplatedby this invention, it is necessary that the apparent porosity be kept toa level of not more than 2%. By adopting the method for the productionof the sintered body of this invention which has been described so far,it is made possible to adjust the apparent porosity at a value in therange of this invention specified above.

The following materials can be used as the raw material powder to beused for the production of the sintered body of this invention.

Electro-molten cordierite, synthetic cordierite, talc, magnesia,magnesium hydroxide, magnesium carbonate, and magnesia spinel are usableas MgO sources, silica is usable as a SiO₂ source, alumina is usable asan Al₂O₃ source, and kaolin and mullite are usable as an Al₂O₃—SiO₂composite source.

The electro-molten cordierite and synthetic cordierite may function alsoas a SiO₂—Al₂O₃ source, the magnesia spinel may function also as anAl₂O₃ source, and the talc may function also as a SiO₂ source.

As the raw material for the MgO source, it is advantageous to supply notless than 70 mass % of the total MgO component of the sintered body withelectro-molten cordierite, synthetic cordierite, or talc. When theproportion occupied by this raw material is set at not less than 70 mass% of the total MgO component, the impartation of a black color can beimproved to a greater extent. Though the mechanism responsible forinducing this phenomenon has not been elucidated fully, it may belogically explained by a supposition that the difference in the crystalphase of the sintered body occurring from the intermediate phase throughthe terminal phase of the sintering process produces an influence.

Also from the standpoint of improving the yield of sintering of articlesin a large shape or in a complicated shape, the use of the raw materialsof electro-molten cordierite, synthetic cordierite, and talc proveadvantageous.

As Li₂O sources, lithium carbonate, lithium oxide, β-spodumene,eucriptite, and petalite powder can be used. From the viewpoint ofimproving sintering properties of products of a large shape and acomplicate shape, the β-spodumene, eucriptite, and petalite powder areparticularly suitable as raw material powders.

As transition element sources, oxides, hydroxides, nitrates, andcarbonates of transition elements, and the metal powders thereof can beused.

EXAMPLE

Now, Examples (Nos. 1–16) of this invention will be described below inconjunction with Comparative Examples (Nos. 17–23).

As raw material powders, magnesia (average particle diameter 0.2 μm),talc (average particle diameter 5 μm), electro-molten cordierite(average particle diameter 3 μm), synthetic cordierite (average particlediameter 2.5 μm), magnesium hydroxide (average particle diameter 0.5μm), magnesium carbonate (average particle diameter 1 μm), lithiumcarbonate (average particle diameter 2 μm), lithium oxide (averageparticle diameter 1 μm), β-spodumene (average particle diameter 5 μm),euriptite (average particle diameter 5 μm), petalite (average particlediameter 4 μm), silica (molten silica, average particle diameter 0.7μm), alumina (average particle diameter 0.3 μm), kaolin (averageparticle diameter 2.5 μm), and mullite (average particle diameter 1 μm)were used. The synthetic cordierite was obtained by mixing magnesia,silica, and alumina powder at a ratio satisfying a theoreticalcomposition of cordierite crystal (2MgO·2Al₂O₅·5SiO₂), and allowing thecomponents to react with one another at 1420° C. for 10 hours therebytransforming the mixture into a cordierite in a granular form. Thegranular cordierite was pulverized prior to use.

As the raw materials for the transition element sources, oxides,hydroxides, nitrates, carbonates, or metal powders of the relevanttransition elements were used.

TABLE 1 Example Chemical composition (mass %) No. Raw material powder toused MgO Al₂O₃ SiO₂ Li₂O X(*1) Y(*2) Coloring auxiliary (*3) 1 Talc ·petalite 13.1 35.0 51.1 0.3 3.71 1.38 Fe₂O₃ 0.3% Mullite · silica CoO0.2% 2 Electro-molten cordierite · silica 9.8 28.4 60.4 0.6 5.67 1.96Fe₂O₃ 0.6% Euciptite · mullite Cr₂O₃ 0.2% 3 Talc · β-spodumene 10.6 36.252.4 0.1 4.87 1.43 Fe₂O₃ 0.7% Kaolin · alumina 4 Electro-moltencordierite · silica 12.6 32.3 54.5 0.4 4.07 1.59 Fe₃O₄ 0.2% petalite ·alumina 5 Talc · kaolin · alumina 15.8 22.4 60.8 0.2 3.72 2.64 Nickelnitrate 0.4% Lithium carbonate · silica Cr₂O₃ 0.3% 6 Syntheticcordierite · silica 9.8 29.5 57.5 2.0 4.23 1.41 Fe₂O₃ 1.0% Lithiumhydroxide · mullite Cr₂O₃ 0.2% 7 Talc · β-spodumene 11.5 33.4 54.2 0.74.23 1.46 Fe₂O₃ 0.2% Alumina · silica · magnesia 8 Electro-moltencordierite · silica 8.2 29.7 60.2 1.4 5.98 1.65 Copper oxide 0.3%petalite · alumina Manganese carbonate 0.2% 9 Electro-molten cordierite· silica 11.3 30.3 57.9 0.2 4.98 1.86 Cr₂O₃ 0.3% Euciptite · mullite 10Talc · petalite · silica 13.0 33.4 51.7 0.4 3.73 1.45 Fe₂O₃ 1.0%Magnesium hydroxide · alumina CoO 0.5% 11 Electro-molten cordierite ·silica 12.3 33.0 53.7 0.6 3.98 1.48 Fe₂O₃ 0.4% β-spodumene · alumina 12Talc · magnesia 13.8 24.2 60.2 0.8 3.90 2.22 Fe₂O₃ 0.7% Lithium · silica· kaolin Cr₂O₃ 0.3% 13 Electro-molten cordierite · silica 15.2 32.0 51.90.1 3.36 1.60 Fe₂O₃ 0.8% Lithium oxide · alumina 14 Talc · β-spodumene9.9 30.1 58.4 1.0 5.09 1.67 Fe₂O₃ 0.3% Alumina · silica Niobium oxide0.3% 15 Synthetic cordierite · silica 8.5 28.9 60.4 1.2 5.98 1.76 Fe₃O₄1.0% β-spodumene · alumina 16 Electro-molten cordierite · silica 10.734.5 54.3 0.0 5.07 1.57 Fe 0.2% Alumina · magnesium hydroxide Cr₂O₃ 0.3%17 Talc · alumina · silica 14.8 36.2 48.2 0.5 2.99 1.22 Fe₂O₃ 0.3%β-spodumene mullite 8 Electro-molten cordierite · talc 18.5 26.3 54.30.2 2.85 2.00 Fe₂O₃ 0.7% Silica · lithium oxide · alumina 9 Talc ·kaolin · silica 16.4 20.9 61.8 0.1 3.72 2.92 Copper oxide 0.5% Lithiumoxide · alumina Cr₂O₃ 0.7% 10 Electro-molten cordierite · silica 12.534.8 52.3 0.4 3.93 1.41 None Lithium oxide · mullite 11 Syntheticcordierite · silica 8.5 28.9 60.4 1.2 5.98 1.76 Fe₂O₃ 1.0% β-spodumene ·alumina 12 Talc · petalite 9.3 31.8 56.5 2.4 4.01 1.17 None Lithiumcarbonate · alumina 13 Talc · kaolin 5.7 30.6 58.4 5.3 2.81 0.52 Noneβ-spodumene *1 X = (SiO₂ − 8 × Li₂O)/MgO (mass ratio) *2 Y = (SiO₂ − 8 ×Li₂O)/Al₂O₃ (mass ratio) *3 The amount of a given coloring auxiliary isexpressed in mass % as reduced to the relevant oxide.

TABLE 2 Sintering Thermal expansion method Gas for coefficient atApparent Young's Specific Total Cordierite Example Sintering atmosphereroom temperature porosity modulus rigidity reflectivity crystal phaseNo. temperature (*1) 10⁻⁶/° C. % GPa GPa · cm³/g % ratio % 1 Hot press,Argon + 0.14 0 141 52.2 9.8 100 1420° C. hydrogen 5% 2 Gas pressure,Argon 0.11 0 112 48.2 11.2 100 1400° C. 3 HIP, Argon 0.21 0 157 58.8 9.296 1375° C. 4 Normal pressure, Argon 0.01 0.1 134 54.2 11.8 100 1370° C.5 Hot press, Argon + 0.35 0 121 51.5 14.5 91 1420° C. hydrogen 5% 6 Gaspressure, Argon −0.09 0 120 46.5 15.2 96 1420° C. 7 Normal pressure,Argon + 0.14 0.2 129 50.6 12.2 100 1390° C. hydrogen 5% 8 Gas pressure,Argon −0.18 0 108 48.0 15.2 97 1290° C. 9 Normal pressure, Helium 0.040.1 147 59.8 11.2 100 1385° C. 10 Hot press, Argon + 0.05 0 138 53.2 8.9100 1420° C. hydrogen 5% 11 Normal pressure, Argon −0.01 0.1 128 54.410.9 100 1350° C. 12 Hot press, Argon + 0.28 0 118 49.8 9.7 97 1320° C.hydrogen 5% 13 Gas pressure, Argon 0.52 0 185 68.5 10.2 86 1450° C. 14Hot press, Argon 0.18 0 121 50.0 16.1 100 1250° C. 15 Normal pressure,Argon + 0.25 0.1 120 52.0 7.9 98 1300° C. hydrogen 5% 16 Hot press,Argon + 0.29 0 151 56.5 14.6 100 1420° C. hydrogen 5% 17 Hot press,Argon + 1.95 0 141 50.2 38.2 89 1400° C. hydrogen 5% 18 Gas pressure,Argon 1.42 0 145 55.0 19.3 76 1450° C. 19 Normal Pressure Argon 0.98 0.482 40.0 18.5 72 1300° C. 20 Normal pressure Argon 0.15 0.1 117 46.6 29.797 1390° C. 21 Normal pressure Argon + 0.42 5.1 72 36.2 43.5 54 1180° C.hydrogen 5% 22 Normal pressure Air 0.52 0.9 88 39.0 80.2 55 1370° C. 23Normal pressure Air 0.60 2.8 65 32.0 74.6 46 1280° C. *1 + hydrogen 5%:Add a hydrogen gas until a concentration thereof reaches 5 vol. % in theambient gas.

As shown in Table 1, the samples of Example Nos. 1–16 and ComparativeExample Nos. 17–23 were obtained by combining the relevant raw materialpowders in such proportions as form the chemical compositions shown inTable 1, adding 3 mass parts of a resin binder to each of the resultantmixtures, and mixing the produced blends each with water as a solvent inan alumina pot mill for 24 hours. The resultant slurries were each driedand granulated and then formed in a prescribed shape under staticpressure of 1500 kg/cm² (147 MPa). The formed product was heated in airto 500° C. to degrease the resin binder.

The degreased formed product was sintered by a sintering method, in asintering atmosphere, at a sintering temperature mentioned in Table 2.The sintering was performed under a surface pressure of 400 MPa by thehot press method, under a gas pressure of 50 kg/cm² (5 MPa) by the gaspressure sintering method, and under 1500 atmospheres at 1300° C. afterthe step of normal pressure sintering by the HIP method. The sinteringtimes each at a relevant temperature indicated in the tables were 4hours in the normal pressure sintering and the gas pressure sintering,and 1 hour in the hot press sintering method and the HIP sinteringmethod.

The produced sintered bodies were each tested for thermal expansioncoefficient at room temperature, total reflectivity, apparent porosity,Young's modulus, and cordierite crystal phase ratio. The results areshown in Table 2. The total reflectivity was determined in accordancewith JIS K7105. Since the thermal expansion coefficient at roomtemperature requires accurate determination, the determination wascarried out in accordance with JIS R3251 (dual light path Michelson'slaser interference method) for the use in the determination of a thermalexpansion coefficient of a low thermal expansion glass. The apparentporosity was determined by the Archimedes method. The cordierite crystalphase ratio was determined by the X-ray diffraction, on the conditionthat the crystal phase comprising pure cordierite crystals and a crystalphase having a lattice constant altered by a solid solution of Li and atransition element while possessing a diffraction peak of cordieritecalculated was reckoned as a cordierite crystal phase.

Experiment Nos. 1–16 represented working examples of this invention,which invariably produced satisfactory results.

Experiment Nos. 17–23 represented comparative examples.

The sample of No. 17 showed thermal expansion coefficient and totalreflectivity both outside the respective ranges contemplated by theinvention because the SiO₂ composition and the ratio X thereof bothdeviated from the ranges of this invention.

The sample of No. 18 showed thermal expansion coefficient, totalreflectivity, and cordierite crystal phase ratio all outside therespective ranges contemplated by the invention because the MgOcomposition and the ratio X both deviated from the ranges of thisinvention.

The sample of No. 19 showed thermal expansion coefficient, Young'smodulus, total reflectivity, and cordierite crystal phase ratio alloutside the respective ranges contemplated by this invention because theAl₂O₃ composition deviated from the range of this invention.

The sample of No. 20 showed total reflectivity outside the relevantrange contemplated by this invention because it had no transitionelements added.

The sample of No. 21 showed apparent porosity, Young's modulus, specificrigidity, total reflectivity, and cordierite crystal phase ratioinvariably outside the respective ranges contemplated by this inventionbecause the sintering temperature was below the lower limit specified bythis invention.

The sample of No. 22 showed the ratio Y outside the range fixed by thisinvention and was sintered in air without any addition of a transitionelement. Specifically, the method disclosed in the publication ofJP-A-61-72,679 was performed on this sample. As a result, this sampleshowed total reflectivity widely deviating from the range fixed by thisinvention and Young's modulus, specific rigidity, and cordierite crystalphase ratio also falling outside the respective ranges contemplated bythis invention.

The sample of No. 23 showed MgO and Li₂O compositions and ratios X and Youtside the respective ranges fixed by this invention and was sinteredin air without adding any transition element. Specifically, the methoddisclosed in the publication of JP-A-10-53,460 was performed on thissample. As a result, the total reflectivity of the sample widelydeviated from the range fixed by this invention and apparent porosity,Young's modulus, specific rigidity, and cordierite crystal phase ratiothereof were outside the respective ranges contemplated by thisinvention.

INDUSTRIAL APPLICABILITY OF THE INVENTION

By this invention, it is made possible to realize a black low thermalexpansion ceramic sintered body having a black tone, manifesting verylow thermal expansion at room temperature, and abounding in rigidity andspecific rigidity and a method for the production thereof. For the firsttime, therefore, this invention has materialized the qualities that havebeen demanded by such members requiring a black tone in devices usinglaser beam and ultraiviolet light while securing dimensional accuracyand stability necessary for the production units for recent highlyintegrated semiconductors and miniaturized magnetic heads.

1. A method for the production of a black low thermal expansion highspecific rigidity ceramic sintered body, comprising: forming thesintered body in an atmosphere of a non-oxidizing gas at a temperaturein the range of 1200–1500° C., said black low thermal expansion highspecific rigidity ceramic sintered body, comprising: having a thermalexpansion coefficient of not more than 0.6×10⁻⁶/° C. in absolute valueat room temperature, a modulus of elasticity (Young's modulus) of notless than 100 GPa, a specific rigidity (Young's modulus/specificgravity) of not less than 40 GPa·cm³/g, and assuming a black tone, saidblack low thermal expansion high specific rigidity ceramic sintered bodyhaving a chemical composition comprising: 8.0–17.2 mass % of MgO,22.0–38.0 mass % of Al₂O₃, 49.5–65.0 mass % of SiO₂, a total of 0.1–2mass % of one or more transition elements as reduced to oxides, 0–2.5mass % of Li₂O, and having the mass ratios satisfy the relationships of(SiO₂−8×Li₂O)/MgO≧3.0 and (SiO₂−8×Li₂O)/Al₂O₃≧1.2.
 2. A black lowthermal expansion high specific rigidity ceramic sintered body,comprising: having a thermal expansion coefficient of not more than0.6×10⁻⁶/° C. in absolute value at room temperature, a modulus ofelasticity (Young's modulus) of not less than 100 GPa, a specificrigidity (Young's modulus/specific gravity) of not less than 40GPa·cm³/g, and assuming a black tone.
 3. A black low thermal expansionhigh specific rigidity ceramic sintered body according to claim 2,wherein the total reflectivity of the sintered body is not more than 17%at a wavelength of light in the range of 200–950 nm.
 4. A black lowthermal expansion high specific rigidity ceramic sintered body accordingto claim 2, wherein the apparent porosity of the sintered body is notmore than 2%.
 5. A black low thermal expansion high specific rigidityceramic sintered body according to claim 2, wherein not less than 80vol. % of the crystal phase of the sintered body is a crystal phase ofcordierite.
 6. A black low thermal expansion high specific rigidityceramic sintered body according to claim 2, wherein the thermalexpansion coefficient is not more than 0.3×10⁻⁶/° C. in absolute valueat room temperature.
 7. A black low thermal expansion high specificrigidity ceramic sintered body according to claim 2, wherein the modulusof elasticity is not less than 120 GPa and the specific rigidity is notless than 50 GPa·cm³/g.
 8. A black low thermal expansion high specificrigidity ceramic sintered body, comprising: having a thermal expansioncoefficient of not more than 0.6×10⁻⁶/° C. in absolute value at roomtemperature, a modulus of elasticity (Young's modulus) of not less than100 GPa, a specific rigidity (Young's modulus/specific gravity) of notless than 40 GPa·cm³/g, and assuming a black tone, said black lowthermal expansion high specific rigidity ceramic sintered body having achemical composition comprising: 8.0–17.2 mass % of MgO, 22.0–38.0 mass% of Al₂O₃, 49.5–65.0 mass % of SiO₂, a total of 0.1–2 mass % of one ormore transition elements as reduced to oxides, 0–2.5 mass % of Li₂O, andhaving the mass ratios satisfy the relationships of(SiO₂−8×Li₂O)/MgO≧3.0 and (SiO₂−8×Li₂O)/Al₂O₃≧1.2.